1. Field of the Invention
The present invention relates to a thermal printing system, and more particularly, to a thermal head supporting means for a thermal printing system, having features of light weight, flexibility, ready for adjustment and is especially suitable for an open-frame type thermal printing system, namely, such one the frame of which is separated into two pieces of an upper frame and a lower frame, and the former is hinged onto the latter so as to be swingably movable upward and downward and, further, superior in accuracy of printing.
2. Description of the Prior Art
A thermal printing system, generally, functions to print alphanumeric and/or image information on a paper sheet by melting thermaly fusible ink applied on the surface of a thermal printing ribbon or ribbons, by means of passing both a paper sheet and thermal printing ribbon being overlapped, through a gap between a thermal head and a platen roller, and it has various features such as small size, light weight yet allowing multi-colour printing in a quite a simple way.
In order to make the density of the printed letters or images more uniform and to improve the cleanliness by using a thermal printing system of this kind, it is very important to satisfy following two requirements, that is, to make a depression force between a thermal head and a mating platen roller in the direction transverse to the width of paper sheet more uniform, and at the same time to perfectly align the axis line of the platen with the location of each of the heating elements in the thermal head.
Because of this, it has been required for most thermal printing system to adjust their depression force in the transverse direction as uniform as possible by means of an adjusting screw or screws, while to align the axis line of the platen roller with each heating element relying on the dimensional accuracy of the related parts or components.
FIG. 1 is a side view showing the structural feature of the thermal head supporting means of a conventional general type thermal printing system.
In the drawing, a thermal head 50 held by a head holder 31 is depressed on a platen 21, and a thermal printing paper and a printing ribbon being overlapped with each other are passed through the gap between these two members and further fed upward by the platen roller 21.
The head holder 31 is disposed around a head holder shaft 31a rotatably received by the same frame carrying said platen roller 21 and is pulled rightward by a depression spring 7 so that the thermal head 50 can be depressed upon the platen roller 21.
The extent of the depression force mentioned above can be adjusted by an adjustment screw 37 disposed on both sides of the head holder 31 so as to be uniform in the direction transverse to the direction of the paper sheet 5.
In the drawing, numeral 38 denotes a sensor for detecting the position of a printing ribbon 4, numeral 39 denotes a sensor holder, similarly, numeral 37' is a locking screw for fixing the adjustment screw 37 and 41 denotes a heat sink for radiating heat given from the thermal head 50.
However, there found following defects in the aforesaid conventional thermal head supporting means, that is:
(1) Adjustment is required to maintain the depression force uniform between the thermal head and platen roller.
(2) The construction of the supporting means inevitably becomes complicated due to such adjustment means.
In order to obviate such drawbacks as explained above, the inventors of the present invention proposed a prior invention directed to a supporting means for a thermal head which enables the maintaining of a uniform depression force between a thermal head and a platen roller, and filed a patent application [Japanese Patent Application No. Sho 60(1985)-150,686; published as Japanese Patent Laid-Open(Un-examined) Publication No. Sho 62(1987)-0011670].
FIG. 2 is a schematic side view showing the main portion of the above-mentioned thermal head supporting means, wherein a rolled paper sheet 5 payed out from a roller R.sub.1 is fed via a roller R.sub.2 onto the outer periphery of a platen roller 21 and then fed leftward by turning back along the outer periphery of the platen roller 21.
On the other hand, an ink ribbon 4 applied with thermally fusible printing ink is also payedout from a roller R.sub.3 and overlapped with the paper sheet 5 on the outer periphery of the platen roller 21 and then taken up by a take-up roller 39.
The thermal head supporting means as explained above and shown in FIG. 3A and 3B is composed of following three main components:
(1) a hinge shaft 40 disposed by transversely passing through both outer side frames 30a and 30b and placed in parallel with the platen roller 21,
(2) a head holder 31 fitted around the outer periphery of the platen roller 21 through a rolling bearing 35 and is positioned halfway between the hinge shaft 40 and the platen roller 21 and holds a thermal head 50 at the side confronting with the platen roller 21, and
(3) a pair of coil springs 7a and 7b attached at both axial ends of the head holder 31 so as to resiliently urge the head holder 31 against the shaft of the platen wheel 21.
Having a construction as explained above, the head holder 31 can be swung in a see-saw like motion around the hinge shaft 40 via the rolling bearings 35.
In addition, the depression force exerted between the platen roller 21 and the thermal head can be made uniform over the entire width of the printing paper, and by virtue of this prior type thermal head supporting means, an entirely uniform depression force can be assured between the thermal head and the platen roller with no particular adjustment.
It is to be noted, however, that the head holder 31 of the prior invention consists of,
(1) a frame member 32 having its transverse width almost the same as that of the outer frames 30a and 30b and its configuration like a squared U shape seen in plan view, and
(2) a supporting member 33 carrying a ball bearing 35 on the opened face side.
Both side walls 32a, 32b and a transverse connecting bar 32c of the frame member 32 are integrally formed by such means as aluminum alloy die casting technique or the like, and these members themselves have a considerable volume together with a heavy weight.
Moreover, a supporting member 33 of considerably large volume is assembled with both the side walls 32a and 32b at the opened left side end by means of a hinge shaft 40 which passing through the two side walls 32a, 32b and the supporting member 33 interposed between the two side frames.
Consequently, supporting means of the inventors' aforesaid prior invention, consisting of the squared U shaped frame, the hinge shaft 40 and the supporting member 33, constitute a rigid construction as a kind of panelled wall and yet being large both in volume and weight.
FIG. 3C is a schematic plan view showing general shape and construction of the supporting means as mentioned above.